The invention relates to a process for the preparation of a hydrocarbon mixture having a Ramsbottom Carbon Test value (RCT) of (a) %w and an initial boiling point of T.degree. C.
The RCT is an important parameter in the assessment of the suitability of heavy hydrocarbon oils as feedstocks for catalytic conversion processes, such as catalytic cracking, carried out in the presence or absence of hydrogen, for the preparation of light hydrocarbon distillates, such as gasoline and kerosine. According as the feed has a higher RCT, the catalyst will be deactivated more rapidly in these processes.
Residual hydrocarbon mixtures, such as residues obtained in the distillation of a crude mineral oil and asphaltic bitumen obtained in the solvent deasphalting of the said distillation residues generally have too high an RCT to be suitable without previous treatment for use as feeds for the afore-mentioned catalytic conversion processes. Since the RCT of residual hydrocarbon oils is mainly determined by the proportion of asphaltenes present in the mixtures, a reduction of the RCT of these mixtures can be obtained by reducing the asphaltenes content. Basically, this may be achieved in two ways. Part of the asphaltenes may be separated from the oil by solvent deasphalting, or part of the asphaltenes may be converted by subjecting the oil to a catalytic hydrotreatment. For the reduction of the RCT of distillation residues the latter method is preferred, in the first place, because its yield of heavy product with a low RCT is higher and further because, in contrast to the former method, where asphaltic bitumen is obtained as a by-product, it yields a valuable C.sub.5.sup.+ atmospheric distillate as a by-product. In view of the scantiness of the yield produced when the former method is used, for the preparation of a low-RCT heavy product from asphaltic bitumen, the latter method is the only one eligible. A drawback to the latter method, however, is that it gives rise to the formation of an undesirable C.sub.4.sup.- fraction which, moreover, contributes considerably to the hydrogen consumption of the process.
Applicants have carried out an investigation into the reduction of the RCT through catalytic hydrotreatment of asphaltic bitumen obtained in the distillation residue from a crude mineral oil. This investigation has shown that, according as the catalytic hydrotreatment is carried out under more severe conditions in order to attain a greater RCT reduction, the parameter "C.sub.4.sup.- production per % RCT reduction" (for the sake of brevity hereinafter referred to as "G") at first remains virtually constant (G.sub.c) and subsequently shows a fairly sharp increase. In view of the hydrogen consumption of the process it is important to take care that the RCT reduction is not carried beyond the value corresponding with G=2.times.G.sub.c. This means that in practice there will be a number of cases in which it is undesirable, starting from an asphaltic bitumen obtained in the solvent deasphalting of a distillation residue from a crude mineral oil, to employ nothing but a catalytic hydrotreatment for the preparation of a product from which, after separation of an atmospheric distillate, an oil having an initial boiling point of T.degree. C. and an RCT of (a) %w can be obtained. In those cases there is nevertheless an attractive manner of preparing from this asphaltic bitumen an oil having the aforementioned initial boiling point and RCT. To this end the product obtained in the catalytic hydrotreatment is separated by distillation into an atmospheric distillate and an atmospheric residue having an initial boiling point of T.degree. C. The process may be continued in two ways. First, from the atmospheric residue so much asphaltic bitumen may be separated by solvent deasphalting that a deasphalted atmospheric residue having the desired RCT of (a) %w is obtained. Secondly, the atmospheric residue may be separated by distillation into a vacuum distillate and a vacuum residue and from the vacuum residue so much asphaltic bitumen may be separated by solvent deasphalting that a deasphalted vacuum residue is obtained having an RCT which is such that when this deasphalted vacuum residue is mixed with the previously separated vacuum distillate, an oil is obtained which has the desired RCT of (a) %w. The most attractive balance between yields of: C.sub.4.sup.- fraction, C.sub.5.sup.+ atmospheric distillate, asphaltic bitumen and oil having an initial boiling point of T.degree. C. and an RCT of (a) %w is obtained when the catalytic hydrotreatment is carried out under such conditions that G lies between 1.5.times.G.sub.c and 2.0.times.G.sub.c. When the catalytic hydrotreatment is carried out under such conditions that G&lt;1.5.times.G.sub.c, C.sub.4.sup.- production is still low, but the yield of oil having an initial boiling point of T.degree. C. and an RCT of (a) %w in the combination process is unsatisfactory. When the catalytic hydrotreatment is carried out under such conditions that G&gt;2.0.times.G.sub.c, a high yield of oil having an initial boiling point of T.degree. C. and an RCT of (a) %w in the combination process is still obtained, but is attended with unacceptably high C.sub.4.sup.- production.
Applicants have found that the RCT reductions in the catalytic hydrotreatment, in which for G values are reached which correspond with 1.5.times.G.sub.c and 2.0.times.G.sub.c, are dependent on T, the RCT of the asphaltic bitumen (b %w) and the average molecular weight (M) of the asphaltic bitumen, and are expressed by a numerical relation.